Cold-restraint stress reduces [3H]etorphine binding to rat brain membranes: influence of acute and chronic morphine and naloxone.

[3H]Etorphine binding was characterized in rat brain homogenates depleted of endogenous opioids from animals acutely and chronically treated with morphine or naloxone and either unstressed or subjected to a 3-h restraint period in the cold. There was significant reduction in the number of high-affinity opiate binding sites in brain tissue from stressed as compared to unstressed animals. Despite the fact that the opiate drug regimens used produce marked behavioral and physiological effects, stress-induced opiate receptor depletion was not influenced by the drugs or by withdrawal. The various drug treatments also failed to produce significant changes in opiate receptor site densities or affinities in either stressed or unstressed animals. We propose that persistent activation of opiate receptors by endogenous opioids released during restraint stress leads to receptor 'down-regulation'.